Microfluidic systems typically involve control of fluid flow through one or more microchannels. One class of systems includes microfluidic “chips” that include very small fluid channels and small reaction/analysis chambers. These systems can be used for analyzing very small amounts of samples and reagents and can control liquid and gas samples on a small scale. Microfluidic chips have found use in both research and production, and are currently used for applications such as genetic analysis, chemical diagnostics, drug screening, and environmental monitoring. Although these systems may allow manipulation of small volumes of fluids, additional methods that allow further control and flexibility are needed.